Nonequilibrium Copolycondensation in Homogeneous Systems

Abstract

Copolycondensation is a polycondensation of three or more monomers which under certain process conditions cannot be condensed by themselves. Some systems containing two condensable monomers, such as hydroxy or amino acids, are also thought to be copolycondensation polymers. Since the chains of copolycondensation copolymers contain two or more types of structural units, one can differentiate between them by their degree of polymerization, structure, and composition (microheterogeneity). In copolycondensation, as well as in copolymerization, it is important to relate quantitatively monomer activities and process conditions to the characteristics of the resulting copolymers. Even though this problem was solved 20 years ago [1, 2] in radical polymerization, systematic investigations of quantitative studies have only recently begun on copolycondensation polymers. This can probably be explained by the fact that at different stages of polycondensation investigation, largely equilibrium processes were studied. It is known [3, 4] that in order to prepare high molecular weight products by this method, equimolar monomer mixtures and conversions close to quantitative are required. Li this case the monomer is incorporated into the polymeric chains, and hence the composition presents no problem. As to heterogeneity of composition, the composition distribution of the products of nonequilibrium copolycondensation must be very narrow. This phenomenon suggests with a great degree of accuracy that the same chemical composition of all macromolecules is characteristic of the equilibrium copolycondensation and, generally speaking, does not take place in the co-polymerization. The possibility of a very significant composition heterogeneity occurring in the latter process is due to the fact that at earlier stages the resulting copolymer can appreciably be enriched with one of the monomers and at later stages with the other. In equilibrium copolycondensation, intensive exchange reactions “intermix” the compositions of single macro molecules, hence eliminating composition heterogeneity and leading to a random (statistical) distribution of units in the copolymer chains. Thus the equilibrium copolycondensation yields macromolecules with a statistical unit distribution and almost identical composition corresponding to that of the starting monomer mixture. Apparently the dependence of the statistical characteristics of the copolymer on the process conditions would be of no importance for an equilibrium copolycondensation. However, such a problem becomes rather urgent for a nonequilibrium copolycondensation involving no exchange or destructive reactions [5]. In this case, one can clearly see the difference between homophase and heterophase procedures in which chemical reactions may be complicated by diffusion and adsorption of reagents on the phase interface.